Attempts are being made to develop adoptive immunotherapeutic techniques utilizing the transfer of cells grown in long-term culture in interleukin-2. Techniques for the prolonged growth of cytotoxic and proliferative T cell lines and clones with anti-tumor reactivity have been developed. These cells have been shown to mediate the immunologic rejection of allogafts and syngeneic tumors and attempts to use these cells in the adoptive immunotherapy of mouse and human tumors are in progress. A new class of cytotoxic cells has been described in both the mouse and the human. These lymphokine activated killer (LAK) cells develop selective cytotoxicity for cancer cells following incubation in the lymphokine, interleukin-2. The adoptive transfer of these cells into mice bearing established tumors can mediate the inhibition of pulmonary and hepatic metastases. The systemic administration of interleukin-2 has been shown to enhance immune responses in vivo. In the past year, a new immunotherapeutic trial began studying the effects of adoptive transfer of lymphokine activated killer cells and recombinant IL-2 into patients with advanced cancer. Twenty-one objective responses have been seen in 55 evaluable patients.